Abstract
Traditionally, fisheries management has assumed that all fish within a marine population contribute the same per unit biomass to population growth. However, it has become evident that older, larger individuals may contribute more to the reproductive output and success. Relatively small subpopulations may sustain a large population network, by contributing a reproductive surplus to the larval pool. For the sustainable management of marine fish populations, it is crucial to understand the reproductive contribution of populations and individual fish within populations, how populations are structured and connected, and how populations interact with the environment.
The present work aimed to elucidate how spatial differences in habitat quality affect marine fish populations, with particular focus on spatial variations in population demographics and reproductive output among subpopulations distributed across seascapes. Firstly, I compared demographic parameters, such as the length-frequency distributions, sex ratios, length-at-sex-change, and potential fecundity among five subpopulations of blue cod (Parapercis colias) in the Marlborough Sounds and Tasman Bay (Chapter 2). Secondly, I used stable isotopic values and stomach contents of blue cod subpopulations to investigate the effect of long-term trawling and dredging in Tasman Bay on the trophic niche of blue cod compared to subpopulations inhabiting biogenic reefs in the Marlborough Sounds (Chapter 3). Thirdly, I inferred population structure and movement from multivariate trace element signatures of otoliths from blue cod subpopulations within and among the Marlborough Sounds and Tasman Bay (Chapter 4). To investigate the contribution to the reproductive output and success of individuals within a population, I used proxies for larval fitness and viability to compare cohorts of larvae produced by different aged and sized female sea perch (Helicolenus percoides) (Chapter 5). Lastly, I explored the effect of maternal nutrition, inferred by fatty acid biomarkers, on larval traits on the day of parturition and larval viability and cohorts of larvae produced by female sea perch sampled from two distinct subpopulations from the Otago shelf and Fiordland (Chapter 6).
Three out of the five sampled subpopulations of blue cod from the Marlborough Sounds and Tasman Bay were severely truncated towards a larger proportion of smaller individuals. Sex change, from female to male, occurred at extremely small lengths, resulting in a sex ratio skewed towards a large proportion of small males among four out of five subpopulations. Consequentially, the estimated population batch fecundity was low among these male-dominated subpopulations. Isotopic niches and niche breadths of blue cod subpopulations from the Tasman Bay regions were smaller than those of subpopulations from the Marlborough Sounds. Movement, as inferred by multivariate otolith trace elemental signatures, was low between subpopulations from the Marlborough Sounds and Tasman Bay. However, subpopulations within the Marlborough Sounds likely displayed a source-sink dynamic, where the outer Sounds subpopulation supported the inner Sounds subpopulation with adults. The two subpopulations likely shared a larval source pool.
Older, larger females of sea perch were proportionally more fecund than younger, smaller females. These older, larger females also produced cohorts of larvae with larger oil globules than did younger, smaller females. Cohorts of larvae, with larger oil globules on the day of parturition, had faster growth rates and survived longer in fed treatments, than cohorts of larvae with smaller oil globules. When larval cohorts were compared between subpopulations from the Otago shelf and the inner Fiordland, cohorts produced by females from the Otago shelf had smaller oil globules on the day of parturition than cohorts of larvae produced by females from Fiordland. Among cohorts of larvae produced by females from Fiordland, the concentrations of the essential fatty acids docosahexaenoic acid (DHA) and arachidonic acid (ARA) were higher, and the eicosapentaenoic acid (EPA)/ARA ratio was lower than in cohorts of larvae produced by females from the Otago shelf.
The results of my research indicated that long-term overexploitation in the Marlborough Sounds and Tasman Bay has likely caused sex-change to occur at smaller sizes, resulting in the higher abundance of smaller males, which resulted in lower population fecundity. Smaller trophic niches of blue cod subpopulations from Tasman Bay compared to those of blue cod from the Marlborough Sounds were coincidental with differences in habitat degradation as a result of intensified bottom trawling and dredging within Tasman Bay. Blue cod populations within the Marlborough Sounds showed complex source-sink population structures and likely shared larval source pools, highlighting the importance of considering population structure when designing and implementing management strategies. Older, larger maternal sea perch produced cohorts of larvae that were more viable than those produced by younger, smaller females, emphasising that an old-growth age structure within populations may ensure larval survival and, ultimately, recruitment. Larval viability was not only affected by maternal age and size, but also by maternal nutrition, which resulted in unique visual and osmoregulatory larval adaptations to the low-light, low-productivity environment of the inner Fiordland. Fisheries management should aim to protect population structure, trophic interactions, and critical habitat simultaneously to sustain marine populations effectively.